Memory is one type of integrated circuitry, and may be used in electronic systems for storing data. Memory is usually fabricated in one or more arrays of individual memory cells. The memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information. The stored memory may be non-volatile wherein the memory state is maintained for a considerable period of time and in many instances where power is completely removed from the circuitry. Alternately, the memory may be volatile, requiring to be refreshed (i.e., rewritten), and in many instances multiple times per second.
One type of non-volatile memory is a phase change memory cell. Such memories use a reversibly programmable material that has the property of switching between two different phases, for example between an amorphous, disorderly phase and a crystalline or polycrystalline, orderly phase. The two phases may be associated with resistivities of significantly different values. Presently, typical phase change materials are chalcogenides, although other materials may be developed. With chalcogenides, the resistivity may vary by two or more orders of magnitude when the material passes from the amorphous (more resistive) phase to the crystalline (more conductive) phase, and vice-versa. Phase change can be obtained by locally increasing the temperature of the chalcogenide. Below 150° C., both phases are stable. Starting from an amorphous state and rising to temperature above about 400° C., a rapid nucleation of the crystallites may occur and, if the material is kept at the crystallization temperature for a sufficiently long time, it undergoes a phase change to become crystalline. Reversion to the amorphous state can result by raising the temperature above the melting temperature (about 600° C.) followed by cooling.
In phase change memory, a plurality of memory cells is typically arranged in rows and columns to form an array or sub-array. Each memory cell is coupled to a respective select or access device which may be implemented by any switchable device, such as a PN diode, a bipolar junction transistor, a field effect transistor, etc. The access device is often electrically coupled with, or forms a part of, what is referred to as an access line or word line. A resistive electrode is electrically coupled with the switchable device, and comprises heater material which is configured to heat up upon sufficient current flowing there-through. The phase change material is provided in proximity to the heater material, thereby forming a programmable storage element. The crystallization temperature and the melting temperature are obtained by causing an electric current to flow through the heater material, thus heating the phase change material. An electrode, typically referred to as a bit, digit, or select line, is electrically coupled to the phase change material.